# vapor pressure of one part of binary compound

Let's say we have a binary compound AB in an enclosed container, and let's say there's a tendency to form B-vacancies so that B atoms/molecules escape and form a vapor. Let's assume there's enough of AB that the pressure saturates before AB$$_{1-x}$$ decomposes too much. Would the vapor pressure of B be the same as for the pure element B?

• Usually the formation of vacancies is not accompanied by vapor formation. Now, unless the AB compound is in equilibrium with solid B, then, no, the vapor pressure of B over AB will not be the same as B over B. Jan 10 '19 at 18:51
• @JonCuster - for quite a few oxides, for example, gaseous oxygen evolves when oxygen vacancies form, so I wouldn't say that simultaneous vapor and vacancy formation is a rare case Jan 10 '19 at 20:27
• @voffch - indeed, for oxides that is the case (and is exploited by various oxide sensors since the vacancy concentration affects the conductivity). I usually deal more with metal alloys, where losing constituents to the gas phase is not an issue. Your answer below can be applied to the more general case. Jan 10 '19 at 20:30

No. The equilibrium vapor (gas) pressure is defined by the corresponding equilibrium. When it's just the element $$\ce{B}$$ that evaporates, the equilibrium is $$\ce{B(liquid)<=>B(gas),}$$ and the thermodynamics of evaporation (or sublimation if $$\ce{B}$$ is solid), such as the Clausius–Clapeyron equation, gives you the equilibrium $$p-T$$ relationship.
When you have nonstoichiometric compound $$\ce{AB_{1-x}}$$ which loses $$\ce{B}$$, the process is quite different. Let's imagine purely hypothetical $$\ce{AB}$$, in the crystal structure of which neutrally charged $$\ce{B}$$ vacancies, $$\ce{V^x_B}$$, can be formed. Then, our hypothetical equilibrium between the gas phase and defects in solid can be written in Kröger–Vink notation as $$\ce{B^x_{B}<=>V^x_{B} + B_{gas}}.$$ In this case, until $$\ce{AB_{1-x}}$$ decomposes too much, the equilibrium constant $$K=\frac{p_B\cdot[\ce{V^x_{B}}]}{[\ce{B^x_{B}}]}$$ and its temperature dependence define the "partial pressure of $$\ce{B}$$ - temperature - composition" relationship for $$\ce{AB_{1-x}}$$.